The present invention relates to the field of disc drives having dual-stage actuators including a coarse actuator and a microactuator. More particularly, the present invention relates to methods and systems for providing plant variation compensation for a piezoelectric microactuator in a dual-stage servo of a disc drive.
One key component of any computer system is a device to store data. Computer systems have many different places where data can be stored. One common place for storing massive amounts of data in a computer system is on a disc drive. The most basic parts of a disc drive are an information storage disc that is rotated, an actuator that moves a transducer head to various locations over the disc, and electrical circuitry that is used to write and read data to and from the disc. The disc drive also includes circuitry for encoding data so the data can be successfully written to and retrieved from the disc surface. A microprocessor controls most of the operations of the disc drive as well as passing the data back to the requesting computer and accepting data from a requesting computer for storing to the disc.
The transducer head is typically placed on a small ceramic block, referred to as a slider, that is aerodynamically designed so that it flies over the disc. The slider is passed over the disc in a transducing relationship with the disc. Most sliders have an air-bearing surface (xe2x80x9cABSxe2x80x9d) which includes rails and a cavity between the rails. When the disc rotates, air is dragged between the rails and the disc surface causing pressure, which forces the transducer head away from the disc. At the same time, the air rushing past the cavity or depression in the ABS produces a negative pressure area. The negative pressure or suction counteracts the pressure produced at the rails. The slider is also attached to a load spring which produces a force on the slider that is directed toward the disc surface. The various forces equilibrate so that the slider flies over the surface of the disc at a particular desired fly height. The fly height is the distance between the disc surface and the transducing head, which is typically equal to the thickness of the air lubrication film. This film eliminates the friction and the resulting wear that would occur if the transducing head and the disc were to be in mechanical contact during the disc rotation. In some disc drives, the slider passes through a layer of lubricant rather than flying over the surface of the disc.
Information representative of data is stored on the surface of the storage disc. Disc drive systems read and write information stored on tracks on the storage discs. Transducers, in the form of read/write heads attached to the sliders, located on both sides of the storage disc, read and write information on the storage discs when the transducers are accurately positioned over one of the designated tracks on the surface of the storage disc. The transducer is also said to be moved to a target track. As the storage disc spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto the track by writing information representative of data onto the storage disc. Similarly, reading data from a storage disc is accomplished by positioning the read/write head above a target track and reading the stored material on the storage disc. To write to or read from different tracks, the read/write head is moved radially across the tracks to a selected target track. The data is divided or grouped together on the tracks. Some disc drives have a multiplicity of concentric circular tracks. In other disc drives, a continuous spiral is one track on one side of drive. Servo feedback information is used to accurately locate the transducer head. The actuator assembly is moved to the required position and held very accurately during read or write operations using the servo information.
The actuator is rotatably attached to a shaft via a bearing cartridge which generally includes one or more sets of ball bearings. The shaft is attached to the base of the disc drive, and may also be attached to the top cover of the disc drive. A yoke is attached to the actuator. A voice coil is attached to the yoke at one end of the rotary actuator. The voice coil is part of a voice coil motor (VCM) used to rotate the actuator and the attached transducer(s). A permanent magnet is attached to the base and to the cover of the disc drive. The VCM which drives the rotary actuator comprises the voice coil and the permanent magnet. The voice coil is attached to the rotary actuator and the permanent magnet is fixed on the base. The yoke is generally used to attach the permanent magnet to the base and to direct the flux of the permanent magnet. Since the voice coil sandwiched between the magnet and the yoke assembly is subjected to magnetic fields, electricity can be applied to the voice coil to drive the voice coil so as to position the transducer(s) at a target track.
Two of the ever constant goals of disc drive designers are to increase the data storage capacity of disc drives, and to decrease the amount of time needed to access the data. To increase storage capacity, current disc drives have increased numbers of tracks per inch (TPI). Put simply, current disc drives squeeze more tracks onto the same size disc. Decreasing the amount of time needed to access the data can be thought of as increasing the speed at which data is retrieved. Increasing the speed at which data is retrieved is very desirable. Any decreases in access time increase the speed at which a computer can perform operations on data. When a computer system is commanded to perform an operation on data that must be retrieved from disc, the time needed to retrieve the data from the disc is often the bottleneck in the operation. When data is accessed from a disc more quickly, more transactions can generally be handled by the computer in a particular unit of time.
A rotating disc data storage device uses a servo system to perform two basic operations: track seeking and track following. Track seeking refers to the ability of the disc drive and the servo system to move the read/write transducer head of the disc drive from an initial track to a target track from which data is to be read, or to which data is to be written. The settling of the transducer head at the target track is referred to as seek settling. Track following, which is performed after the head has been aligned with a target track, refers to the ability of the disc drive and the servo system to maintain the read/write head positioned over the target track. Note that, to effectively perform track seeking and track following in a disc drive with increased TPI, the servo open loop bandwidth of the system must also be pushed or increased.
As the areal density of magnetic disk drives continues to increase, more accurate positioning of the read/write head will be needed. It is expected that track density will achieve or exceed 30,000 TPI by the year 2000. At such high density, a servo bandwidth as high as 1,500 to 2,000 Hz is required to suppress high frequency disturbances. Because the servo bandwidth of single-stage, VCM actuators is far below that value, the use of dual-stage actuators for high-bandwidth, high-accuracy positioning has been introduced. In dual-actuator disc drives, a VCM is used as a first-stage actuator to generate a coarse but large displacement, and a microactuator is used as a second-stage actuator to provide fine and fast positioning. In some dual-actuator disc drives, the second-stage actuator is a piezoelectric microactuator that is mounted on an E-block, and uses piezoelectric elements made of a lead-zirconate-titanate material. Such a microactuator system can be referred to as a PZT system. The first-stage and second-stage actuators are controlled by a dual-stage controller.
Ideally, the PZT system of a dual-actuator disc drive will maintain a constant gain at the frequency range of less than about 2,500 Hz with small plant variations, and will have a sway mode that occurs around 7,000 Hz. In practice, however, the resonance control in the mechanical structure of such a PZT system is not robust. In FIG. 1, for example, a graph 100 shows a comparison between frequency response of the ideal PZT model and a typical measured frequency response of an actual PZT system. The frequency response of the actual PZT system includes gain variations and unwanted resonance modes that appear below the frequency of the sway mode. Such gain and frequency variations degrade the performance of the disc drive and, in some cases, cause stability problems. To handle the gain variations and unwanted resonance modes, a conservative controller design with a large uncertainty bound could be used for controlling such PZT systems. Using a conservative controller design, however, requires a tradeoff in terms of sacrificed system performance. A less conservative dual-stage controller design may also be used. However, such a dual-stage controller design requires accurate system identification, and skilled manual tuning, in order to maintain stability and performance for each specific PZT system. Unfortunately, providing accurate system identification, and skilled manual tuning, is particularly tricky and burdensome to achieve in factory environments.
Therefore, an improved method and apparatus for controlling dual-actuator disc drives is needed. There is also a need for a method and apparatus for handling the gain variations and resonance modes that appear in the frequency response of the PZT systems of dual-actuator disc drives. Further, there is a need for a method and apparatus for controlling dual-actuator disc drives without resorting to conservative controller designs with large uncertainty bounds that result in the sacrifice of system performance, and which applies to all dual-actuator disc drives without performance degradation. There is a need for methods to automatically perform accurate system identification of dual-actuator disc drives, and to automatically tune their control, which can be easily and effectively applied even in automated factory environments.
In accordance with one exemplary embodiment of the present invention, a method of providing plant variation compensation for a microactuator in a dual-stage servomechanism of a disc drive includes the steps of performing indirect adaptive filtering to identify plant variation in the microactuator, and tuning a compensator for the microactuator based on the plant variation.
In one embodiment of this method, indirect adaptive filtering is performed using a two-stage process, including a first stage of adaptive modeling for the dual-stage servomechanism and a second stage of generating an indirect model-reference inverse for the microactuator. In another embodiment of this method, the indirect adaptive filtering is performed using a combined process which includes adaptive modeling for the dual-stage servomechanism, and simultaneously generating an indirect model-reference inverse for the microactuator. The indirect model-reference inverse generated by either embodiment can be used as the compensator. The compensator for the microactuator can be implemented as a finite impulse response (FIR) filter. Alternatively, the FIR filter can first be converted into an infinite impulse response (IIR) filter using linear model fitting. In one embodiment, the microactuator is a piezoelectric micro actuator including a piezoelectric element made of a lead-zirconate-titanate material to form a PZT system, and the dual-stage servomechanism includes a coarse actuator such as a voice coil motor (VCM).
In accordance with another embodiment of the present invention, a dual-actuator disc drive includes a base, a disc rotatably attached to the base, a transducer carried in a transducing relation with respect to the disc, a first-stage actuator for providing coarse positioning of the transducer, a second-stage actuator for providing fine positioning of the transducer, and a controller. The controller is coupled to the first-stage and second-stage actuators, and is for monitoring an actual position signal for the transducer and for generating a first and a second control signal for the first-stage and the second-stage actuator, respectively. The controller includes a first and a second control path for the first-stage and second-stage actuator, respectively. The second control path includes a compensator for approximating an ideal second-stage actuator. In one embodiment of this disc drive, the first-stage actuator includes a VCM, and the second-stage actuator includes a PZT system. In alternative embodiments, the compensator includes either an FIR filter or an IIR filter that is based on an indirect model-reference inverse for the second-stage actuator.
In accordance with another embodiment of the invention, a system for providing plant variation compensation for a piezoelectric microactuator in a dual-stage servomechanism of a disc drive includes performing means and tuning means. The performing means is for performing indirect adaptive filtering to identify plant variation in the piezoelectric microactuator. The tuning means is for tuning a compensator for the piezoelectric microactuator based on the plant variation.
These and various other features as well as advantages which characterize the present invention will be apparent to a person of ordinary skill in the art upon reading the following detailed description and reviewing the associated drawings.